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Aquinas on Law

The following streamlined text selections from the Summa Theologiae, Thomas Aquinas’ main work, show his treatment of the philosophical problems of the law. The sections reproduced here contain only the so-called “responsios”; some articles are omitted entirely. Thomas’s approach is very consistent; he has great confidence that reason can function as a guide to guide to ethical decision-making.

Whether law is something pertaining to reason

Law is a rule and measure of actions through which one is induced to act or restrained from acting. Lex, “law,” is derived from ligare, “to bind,” because it binds one to act. The rule and measure of human activity is reason, however, for it is the first principle of human acts. Indeed, it is the function of reason to order to an end, and that is the first principle of all activity according to Aristotle. That which is the first principle in any genus is the rule and measure of that genus, e.g., unity in the genus of number or first movement in the genus of movement. Thus it follows that law is something pertaining to reason.

Whether law is always ordered to the common good

Law pertains to that which is the principle of human acts because it is a rule and measure. Just as reason is the principle of human acts, however, there is something in reason which is principle of all the rest. It is to this that law principally and mostly pertains. The first principle in activity, the sphere of practical reason, is the final end. The final end of human life is happiness or beatitude. Thus law necessarily concerns itself primarily with the order directing us toward beatitude.

Furthermore, since each part is ordered to the whole as imperfect to perfect, and since each single man is a part of the perfect community, law necessarily concerns itself particularly with communal happiness. Thus Aristotle, in defining legal matters, mentions both happiness and the political community, saying, “We term ‘just’ those legal acts which produce and preserve happiness and its components within the political community.” For the state is a perfect community, as he says in his Politics.

In any genus, that which is called “most of all” is the principal of everything else in that genus, and everything else fits into the genus insofar as it is ordered to that thing. For example, fire, the hottest thing, is cause of heat in mixed bodies, which are said to be hot insofar as they share in fire. Thus, since law is called “most of all” in relation to the common good, no precept concerning action has the nature of law unless it is ordered to the common good.

Whether anyone can make laws

Law principally and properly seeks the common good. Planning for the common good is the task of the whole people or of someone ruling in the person of the whole people. Thus lawmaking is the task of the whole charge of the whole people; for in all other matters direction toward an end is the function of him to whom the end belongs.

Whether promulgation is an essential part of law

Law is imposed on others as a rule and measure. A rule and measure is imposed by being applied to those who are ruled and measured. Thus in order for a law to have binding power – and this is an essential part of law – it must be applied to those who ought to be ruled by it. Such application comes about when the law is made known to those people through promulgation. Thus such promulgation is necessary if a law is to have binding force.

Thus from the four preceding articles we arrive at a definition of law: Law is nothing other than a certain ordinance of reason for the common good, promulgated by him who has care of the community.

Whether there is an eternal law

A law is nothing more than a dictate of the practical reason emanating from a ruler who governs some perfect community. Assuming that the world is ruled by divine providence, however, it is clear that the whole community of the universe is governed by divine reason. Thus the very idea of the governance of all things by God, the ruler of the universe, conforms to the definition of a law. And since, as we read in Proverbs 8:23, the divine reason’s conception of things is eternal and not subject to time, a law of this sort can be called eternal.

Whether there is a natural law in us

Since law is a rule or measure, it can be in something in two ways: As that which regulates and measures, or as that which is regulated and measured, for insofar as something participates in a rule or measure it is itself regulated and measured. Since everything subjected to divine providence is regulated and measured by eternal law, it is clear that everything participates in the eternal law in some way. That is, everything inclines to its own proper acts and ends because such an inclination is impressed on it through eternal law.

The rational creature is subject to divine providence in a more excellent way than other beings, however, for he is a participant in providence, providing both for himself and for others. Thus he is a participant in that eternal reason through which he has a natural inclination to his proper act and end, and this participation of the rational creature in eternal law is called “natural law.”

Thus the psalmist says, “Offer a sacrifice of justice” (Ps. 4:6) and then, as if someone were asking him what the works of justice are, he adds, “Many say, ‘Who shows us good things?'” He replies, “The light of your countenance is impressed upon us, Lord,” thus implying that the light of natural reason, by which we discern what is good and bad (which is the function of natural law), is nothing else than an impression of the divine light upon us.

Thus it is clear that natural law is nothing other than the rational creature’s participation in the eternal law.

Whether there is a human law

Law is a certain dictate of practical reason. The process is the same in the case of practical and speculative reason. Each proceeds from certain premises to certain conclusions. Accordingly it must be said that, just as in speculative reason we draw from naturally known, in demonstrable principles the conclusions of various sciences, and these conclusions are not imparted to us by nature but discovered by the work of reason, so it is that human reason starts from the precepts of natural law as from certain common and in demonstrable premises, proceeding from them to more particular determinations of certain matters.

These particular determinations devised by human reason are called “human laws,” provided that all the other conditions included in the definition of “law” are observed. Thus Tully says that “justice took its start from nature, then certain things became customary because of their usefulness. Later the things which started in nature and were approved by custom were sanctioned by fear and reverence for the law.”

Whether it was necessary that there should be a divine law

It was necessary for the direction of human life that, beyond natural and human law, there should be a divine law. There are four reasons for this need.

First, it is through law that man is directed to the acts which are proper to him in view of his ultimate end. If man were ordered only to an end which did not exceed his natural faculties, it would not be necessary for him to have any rational direction beyond natural law and that human law derived from it. Since, however, man is ordered to the end of eternal beatitude, which exceeds natural human faculties, it was necessary that he be directed to this end by divinely-given law in addition to natural and human law.

Second, due to the uncertainty of human judgment, particularly regarding contingent and particular things, there tend to be differing judgments regarding human acts, from which proceed diverse and contrary laws. Therefore, in order for man to be secure in the knowledge of what should be done and what avoided, it was necessary that his acts be directed by a divinely-given law which cannot err.

Third, man can make laws in those areas where he is competent to judge. His judgment does not extend to interior acts which lie hidden, however, but only to exterior acts which are apparent. Nevertheless, perfect virtue involves righteousness in both. Thus human law was unable to curb and direct internal acts sufficiently, and it was necessary that divine law supervene in this task.

Fourth, as Augustine says, human law cannot punish or prohibit all things that are evil, for in the process of removing evil it would also eliminate much that is good and impede the advancement of the common good, thus hurting society. Thus, in order to leave no evil unprohibited and unpunished, a divine law, through which all sins are punished, had to intervene.

These four causes are touched upon in the Psalm, where it is said (Ps. 18:8), “The law of the Lord is unspotted” (that is, permitting no foulness of sin), “converting souls” (because it directs not only exterior but also interior acts), “the testimony of the Lord is sure” (due to certainty of truth and rectitude), “giving wisdom to little ones” (insofar as it orders man to a supernatural and divine end).

Whether the eternal law is the supreme pattern of all things existing in the mind of God

Just as the pattern of something made by an artist pre-exists in the artist’s mind, so in the mind of any governor there pre-exists a pattern of the things to be done by his subjects. Moreover, just as the pattern of things to be made through art is called the art or exemplar, so the governors pattern for the activity of his subjects takes on the nature of law provided that the other above-mentioned
characteristics of law are also present.

God, through his wisdom, is creator of all things and related to them as artist to work of art. He is also governor of all actions and activities found in individual creatures. Thus, just as the divine wisdom, insofar as all things were created by it, has the character of art, exemplar or idea, so also divine wisdom as moving all things to their proper end has the character of law. Accordingly the eternal law is nothing other than the pattern of divine wisdom according to which it directs all acts and motions.

Whether the eternal law is known to all

A thing can be known in two ways: First, in itself; second, in its effect, in which some likeness of it is found, just as someone who does not see the sun in its substance may at least know it by its rays. Thus it must be said that only God and the blessed who see God in his essence can know the eternal law as it is in itself; yet every creature knows it according to some of its greater or lesser radiations.

Every knowledge of truth is a certain radiation of and participation in the eternal law, which is the unchangeable truth, as Augustine says. Everyone knows the truth to some extent, since at least the common principles of natural law are available to him. As for the rest, people know it in greater or lesser degree and thus know more or less of the eternal law.

Whether all law is derived from the eternal law

Law is a certain plan directing acts to their end. Wherever movers are ordered to one another, the power of the second mover must be derived from that of the first, since the second mover operates only insofar as it is moved by the first. We see the same thing in all governors. The plan of government is derived by the secondary governors from the primary governor, just as the plan of what is to be done in a state derives from the king through his command to lesser administrators. It is the same in construction, where building plans descend from the architect to the lesser craftsmen who work with their hands.

Therefore, since the eternal law is a plan of government in the supreme governor, all plans of government in lesser governors must be derived from eternal law. All laws besides the eternal law are plans of this sort devised by inferior governors. Thus all laws are derived from eternal law insofar as they participate in right reason. That is why Augustine says that “in temporal law nothing is just and legitimate which men have not derived from eternal law.”

Whether the contingent acts of nature are subject to the eternal law

Certain distinctions should be made between human law and eternal law, which is the law of God. Human law extends only to rational creatures subject to man. the reason for this is that law governs the actions of those who are subject to the government of someone. Thus no one, properly speaking, imposes a law on his own acts. Whatever is done regarding the use of irrational things subject to man is done by man himself moving these things, since these irrational creatures do not move themselves but are moved by others. consequently man cannot impose law on irrational creatures, no matter how thoroughly these creatures may be subjected to him. He can impose law on rational creatures who are subject to him, however, insofar as by his precept or command he impresses on their minds a rule which becomes a principle of action.

Just as man, by such precepts, impresses a kind of inward principle of actions on whoever is subject to him, so God imprints on the whole of nature the principles of his own proper actions, and thus he is said to command all of nature. As the psalmist says, “He has made a decree which will not pass away” (Ps. 148:6). For this reason all the movements and activities of nature are subject to eternal law. Thus irrational creatures are subject to eternal law inasmuch as they are moved by divine providence, though not by understanding of divine commandment as in the case of rational creatures.

Whether all human affairs are subject to eternal law

A thing can be subject to eternal law in two ways: First, insofar as it participates in the divine law by way of knowledge; second, through acting and being acted upon, insofar as it participates in the divine law as an interior motive principle. Irrational creatures are subject to eternal law in this second way, as was said above.

Since a rational creature, in addition to what it shares in common with all creatures, has some special characteristics precisely because it is rational, it is subject to the eternal law in both ways. It has some knowledge of the eternal law, yet at the same time there is implanted in every rational creature a natural inclination to that which is consonant with eternal law. As Aristotle says, “We are naturally adapted to receive the virtues.”

Each manner of participating in the eternal law is imperfect and corrupted in the wicked, for in them the natural inclination to virtue is corrupted by vicious habits and the natural knowledge of good is overshadowed by passions and sinful habits. Each manner is found in a more perfect condition in the good, for in them natural understanding of the good is supplemented by knowledge through faith and wisdom, while natural inclination toward good is supplemented by the inner power of grace and virtue.

Thus the good are perfectly subject to eternal law in the sense that they always act according to it. The evil are also subject to eternal law. They are imperfectly subject to it as far as their own actions are concerned, since they know the good imperfectly are imperfectly inclined to it. Nevertheless, this deficiency in their action is made up by the way they are acted upon, for they suffer the penalty decreed by the eternal law for those who do not fulfill its commandments.

Whether natural law contains many precepts or only one

The precepts of natural law are related to practical reason as the first principles of demonstration are to speculative reason. Both are self-evident principles.

A thing is said to be self-evident in two ways, in itself and for us. A proposition is said to be self-evident because the predicate is contained in the definition of the subject; yet it will not be self-evident to someone who does not know the definition of the subject. Thus the proposition “man is rational” is self- evident because “rational” is part of the definition of man,” but it is not self-evident to anyone who does not know what a man is.

Thus, as Boethius says, “Certain axioms or propositions are self-evident to all,” and these are propositions the terms of which are understood by all, such as “the whole is greater than the part” or “things equal to a third thing are equal to one another.” On the other hand, some propositions are self-evident only to the wise, who understand what the words mean. Thus, to anyone who knows that an angel is not a body, it is self-evident that an angel is not circumscriptively present in a place; yet that is not at all self-evident to the uneducated, who do not understand.

There is, however, a certain order to those things which fall within the apprehension of man. The first thing which falls within his apprehension is being, the notion of which is included in everything he apprehends. Thus the first in demonstrable principle, that the same thing cannot be simultaneously affirmed and denied, is based on the very notion of being and non-being, and all other principles are founded on this one, as Aristotle says.

Just as being is the first thing which falls within the apprehension as such, so good is the first thing to fall within the apprehension of practical reason, which is ordered to activity. For every agent pursues an end which is perceived as good. Thus the first principle for practical reason is a definition of the good, namely that “the good is that which all men seek after.” Thus the first precept of the law is that “good is to be pursued and performed, evil avoided.” On this principle is founded all others in natural law. Thus whatever practical reason recognizes to be good for man pertains to the precepts of natural law.

Since from this perspective the good is defined as an end to be pursued, while evil is defined as what is contrary to that end, reason naturally sees as good and therefore to be pursued all those things to which man has a natural inclination, while it sees the contraries of these things as evil and therefore to be avoided.

Thus the order of precepts in natural law follows the order of natural inclinations. First of all, within man there is an inclination to good according to the nature he shares in common with all substances. Every substance seeks to preserve its own being according to its nature. Because of this inclination, all the things through which man’s life is preserved belong to natural law.

Second, within man there is an inclination to certain more special things according to the nature he has in common with other animals. Because of this inclination, those things which nature has taught to all animals are said to be part of natural law. This would include sexual intercourse, education of offspring, and the like.

Third, within man there is an inclination to good according to his own particular nature as rational. For example, he has a natural inclination to know the truth about God and to live in society. Inclinations of this sort also pertain to natural law, and thus it is part of natural law that man should shun ignorance, avoid offending those with whom he has to live, etc.

Whether natural law is the same for all

Those things to which man is naturally inclined pertain to natural law. Among such inclinations is one particular to man: The inclination to act according to reason. Reason characteristically proceeds from the general to the particular, as Aristotle says. Speculative reason differs from practical reason in this respect, however. Since speculative reason deals primarily with necessary things which cannot be otherwise than they are, both its general premises and its particular conclusions are unerringly true. Practical reason, however, deals with contingent matters involving human activity. Thus, even if the general premises are necessary, the more we descend to particulars the more frequently we discover defects.

Thus in speculative matters the same truth is found in premises and in conclusions, although the truth of the conclusions is not recognized by all, but only that of the premises, which are called “common notions.” In matters of actions, however, truth or practical rectitude is not always the same in particular instances, but only in general premises. Moreover, even when there is the same rectitude in particular cases, it is not equally recognized by all.

Thus it is obvious that in the case of general premises there is always the same truth or rectitude, whether one is dealing with speculative or practical reason. In the case of speculative reason, particular conclusions are true in all cases, though their truth may not be equally recognized by all. A triangle always has three angles the sum of which is equal to two right angles, even though everyone does not know this. In the case of practical reason, however, particular conclusions are not true in every case, nor, even when they are, is their truth equally recognized.

It is invariably correct that we should act according to reason. It follows from this premise that goods held in trust should be returned to their owners, and such is, in fact, usually the case; yet some situations might occur in which it was harmful and therefore irrational to return goods held in trust. For example, the person might request his goods because he wanted to use them to attack his country. The more one descends to particular cases, the more the general rule admits exceptions, and one finds oneself stipulating that goods held in trust must be restored with such and such a guarantee or in such and such a way; for the more conditions are added, the more ways one finds in which the general rule can fail and the less helpful the general rule by itself becomes in deciding whether or not the goods should be returned.

Thus it must be said that in its general premises natural law is always the same in itself and is commonly seen to be such. In the particular determinations drawn from those general premises, natural law is the same in most cases and is so perceived by most people; yet in a few cases it can fail in itself because of particular impediments (just as the natural processes of generation and decay occasionally fail because of impediments), and it can also fail to be recognized as true because the reasons of some are distorted by passion, evil habits, or evil natural disposition. Thus Julius Caesar remarks that the Germans once did not regard theft as evil.

Whether natural law can be changed

There are two ways in which one can speak of natural being changed. The first is by something being added to it. Nothing would prevent it from being changed in this way, for many things useful to human life have been added to natural law, both by divine law and by human law.

The second way is by subtraction, when something which used to be part of natural law ceases to be such. The first principles of natural law are absolutely immutable. Its secondary precepts, which we have described as certain particular conclusions close to first principles, cannot be changed in such a way that its application in the majority of cases is altered, but in some few cases it can be changed in some particular, due to some special causes impeding the normal observance of such precepts.

Whether it was useful for human laws to be made by man

There is a certain aptitude for virtue in man, but the perfection of that virtue must be achieved through training. In the same way we see that industry aids man in achieving necessities like food and clothing; for he has the beginnings of these things from nature – that is, nature gives him reason and a pair of hands – but not the complete product, as is the case with other animals to whom natures gives food and clothing. It is hard for a single man to gain such training by himself, for the perfection of virtue consists in withdrawing from undue pleasures, to which men are particularly prone. This is particularly true of the young, who are also more easily trained. Thus the training through which men come to virtue needs to be received from someone else.

In the case of those young people who are inclined to acts of virtue by natural disposition, by habit, or by divine gift, paternal discipline in the form of friendly reminders will be sufficient. Nevertheless, since some are not easily moved by words because they are depraved and inclined to vice, it is necessary for such to be restrained from evil by force and fear, so that they will at least stop their evil-doing and leave others in peace, or perhaps eventually, by force of habit, be brought to do willingly what they formerly did through fear, thus becoming virtuous. This kind of training, which compels through fear of punishment, is the training of laws.

Thus it was necessary for peace and virtue that there be laws. As Aristotle says, “If man is perfected by virtue, he is the best animal. If he is separated from law and justice, he is the worst.” Such is the case because man can use the weapons of reason, which other animals lack, to satisfy his lust and rage.

Whether every human law is derived from natural law

As Augustine says, “There never seems to have been a law that was not just.” Thus it has the force of law insofar as it is just. In human affairs, something is said to be just insofar as it is right according to the rule of reason. The first rule of reason is, however, natural law. Thus every human law has the nature of a law insofar as it is derived from natural law. If it conflicts with natural law in any way, then it is not law but a corruption of law.

But it should be noted that something can be derived from the natural law in two ways: First, as conclusions from premises; second, as determinations of certain generalities. The first way is similar to that in which conclusions are demonstratively derived from premises in the sciences. The second is similar to the way general ideas are given special shape in the arts, as when a builder decides that he will actualize the general form of a house by constructing this or that particular model.

Thus some laws are drawn as conclusions from the general principles of natural law. For example, “you must not commit murder” can be derived as a conclusion from “you must do harm to no one.” Other laws are drawn from natural law by way of determination. For example, natural law requires that he who sins should be punished, but that he be punished by this or that punishment is a determination of natural law. Both are found in human law, but the first sort derive their strength, not only from the fact that they are legally enacted, but also from natural law itself. The second sort derive their strength only from human law.

Whatever is directed to an end should be proportioned to that end. The end of law is the common good, because, as Isidore says, “Law should be written, not for private gain, but for the general welfare of the citizens.” Thus laws should be proportioned to the common good.

This good is comprised of many things, and thus law should take many different persons, occupations and situations into account. A political community is composed of many people and its good is secured through many actions. Nor is it created to last a short time, but for a very long time and through generations of citizens, as Augustine says.

Whether it is the business of human law to restrain all vice

Law is established as a certain rule or measure of human actions. A measure ought to be of the same type as the thing measured, as Aristotle observes, for different things have different standards, Thus laws should be imposed on men: according to their condition, because, as Isidore says, “Law should be possible according to the customs of the land.”

The power or possibility of action springs from an internal habit or disposition. Actions that are possible to a virtuous man are not possible to him who lacks the habit of virtue, any more than a boy can do all that a grown man can do. Thus the same law is not imposed on adults and children alike, and many things permitted to children are punished or at least blamed when performed by adults. Similarly, many things are permitted to men imperfect in virtue which would not be tolerated in more virtuous men.

Human law is imposed on the multitude, a part of which is composed of men imperfect in virtue. Thus all the vices from which the virtuous abstain are not punished by human law, but only the more grievous ones which most people can avoid, and especially those which can hurt others, without the prohibition of which human society could not be preserved. Thus homicide, theft and the like are prohibited by human law.

Whether human law prescribes all virtuous acts

Virtues are differentiated according to their objects. These objects all involve either the private good of an individual or the common good of the multitude. Thus an act of courage may be performed to preserve one’s city or to preserve the rights of a friend. It is the same with other virtues.

Law, however, is ordered to the common good. Thus there is no virtue the acts of which cannot be prescribed by law. Nevertheless, law does not prescribe every act of every virtue, but only those which are ordained to the common good, either immediately as when things are done directly for the common good, or mediately as when a lawgiver prescribes certain things pertaining to good discipline through which the citizens will be trained to preserve peace and justice.

Whether human law binds a man’s conscience

Human laws are either just or unjust. If they are just, they have the power to bind our conscience because of the eternal law from which they are derived. As Proverbs says, “Through me kings reign and lawmakers decree just laws” (Prov. 8:15).

Laws are said to be just either because of their end, when they are ordained to the common good; or because of their author, when the law does not exceed the power of the lawmaker; or because of their form, when burdens are distributed equitably among subjects for the common good. For since a man is part of the multitude, whatever he is or has belongs to the multitude as a part belongs to the whole. Thus nature inflicts harm on a part in order to save the whole. Accordingly laws which inflict burdens equitably are just, bind the conscience, and are legal laws.

Laws are unjust in two ways: First, they may be such because they oppose human good by denying the three criteria just mentioned. This can occur because of their end, when a ruler imposes burdens with an eye, not to the common good, but to his own enrichment or glory; because of their author, when someone imposes laws beyond the scope of his authority; or because of their form, when burdens are inequitably distributed, even if they are ordered to the common good. Such decrees are not so much laws as acts of violence, because, as Augustine says, “An unjust law does not seem to be a law at all.” Such laws do not bind the conscience, except perhaps to avoid scandal or disturbance, on account of which one should yield his right. As Christ says, “If someone forces you to go a mile, go another two with him; and if he takes your tunic, give him your pallium” (Mtt. 5:40f.).

Second, laws may be unjust because they are opposed to the divine good, as when the laws of tyrants lead men to idolatry or to something else contrary to divine law. Such laws must never be observed, because “one must obey God rather than men” (Acts 5:29).

Whether someone subject to the law can act outside the letter of the law

All law is ordered to the common well-being of men and gains the force of law from precisely that fact. To the degree that it fails in accomplishing this end, it loses its binding force. Thus the Jurist says, “No reason of law or advantage of equity allows us to interpret harshly and render burdensome those healthy measures which were originally enacted for man s welfare.”

It often happens that a law aimed at the general welfare is useful in most cases and yet on occasion is very harmful. Because a legislator cannot foresee all possible individual cases, he promulgates a law which fits the majority of cases, having the common good in mind. If a case emerges in which the law is harmful to the common good, it should not be observed. For example, if a law says that the gates of a certain besieged city should remain closed, such a law is beneficial to the city in most cases; yet if the enemy is pursuing some of the very citizens by whom the city is defended, refusal to open the gates and let them in would be harmful to the city. In such cases, the gates should be opened despite the letter of the law, in order to attain the common good intended by the legislator.

Note, though, that if obedience to the letter of the law involves no immediate danger calling for instant remedy, not everyone is competent to decide what is good or bad for the city, but only the leaders, who have authority to dispense with the law in such cases. If it is indeed a matter of immediate danger allowing no time to consult a superior, such necessity carries its own dispensation, for necessity knows no law.

Whether human law should be changed in any way

Human law is a dictate of reason by which human actions are directed. Thus change in law has a twofold source: One on the part of reason, the other on the part of the men whose actions are regulated by law.

On the part of reason, it can be changed because it seems natural for human reason to advance gradually from the imperfect to the perfect. Thus we see in the speculative sciences that the early philosophers produced imperfect teachings which were later improved by their successors. So also in the practical realm, those who first tried to discover what was beneficial for human community, being unable to think everything through by themselves, created imperfect situations which were lacking in many ways. These institutions were then altered by subsequent lawmakers, producing institutions which departed from the common good in fewer instances.

On the part of the men whose activities are regulated by law, a law is rightly changed when there is a change in the conditions of men, for different things are expedient in accordance with different conditions. Augustine offers the following example: “If the people are moderate, responsible, and careful guardians of the common good, it is proper to enact a law allowing them to choose magistrates through whom the commonwealth can be administered. If, however, in time the people become corrupted and sell their votes, entrusting the government to scoundrels and criminals, they forfeit their power to elect public officials and the right devolves upon a few good men.

Whether human law should always be changed when something better is possible

It is right to change human law if such a change is conducive to the common good. Nevertheless, the very act of changing a law damages the common good to some extent, because custom encourages people to observe the law. Even minor changes seem to be major when they involve a breach of custom. Thus when a law is changed its binding force is diminished insofar as custom is abolished. For this reason, human law should never be changed unless the advantage to the common good resulting from its alteration outweighs the damage done by the change itself.

Such may be the case if some great and evident benefit is derived from the new law, or if some extreme emergency is occasioned by the fact that the existing law is clearly unjust or its observance extremely harmful new laws the benefit to be derived should be evident before one dispenses with a law that was long considered just.